1. Field
The present invention is related to autofocus for ground penetration radar (“GPR”), and more particularly to, autofocus for wide-beam/wide-band GPR.
2. Description of Related Art
Synthetic aperture radar (“SAR”) is used for ground mapping as well as target identification, where a moving platform, e.g., an aircraft, has an imaging radar that sends transmitted signals from different positions along the path of the platform such that the ground (e.g., ground clutter) reflects signals that are received by the imaging radar.
To achieve high azimuth resolution in SAR image formation, the slant range history of the processed target must be highly accurate. However, high azimuth resolution is not achieved for most SAR systems, since a costly high performance navigation system would be required, as well as accurate knowledge of the terrain elevation. One viable solution is to include an autofocus process during or after SAR processing for improving the image quality.
However, though there are some autofocus solutions for various narrow band and narrow beam SAR systems, the autofocus solution for GPR remains a challenging problem due to several factors. First, the autofocus solution is highly spatially varying within the image frame. For a narrow beam SAR, one autofocus solution is mostly applicable to the entire image. But, for a wide beam SAR, one autofocus solution may only be applicable to a small image block. This is because the phase error caused by the navigation error is angle dependent, as illustrated in FIGS. 2 and 3.
As show in FIG. 2, the distance from the target 200 to the actual aircraft path 202 may deviate from the knowledge aircraft path 204 by a range error 206.
As shown in FIG. 3,
dRA=R′A−RA,
dRB=R′B−RB,
dRA≠dRB,
where dRA is the difference between the distance from the radar actual position 30 to target A and the distance from the radar knowledge position to target A, and where dRB is the difference between the distance from the radar actual position 30 to target B and the distance from the radar knowledge position to target B. Here, dRA and dRB are not equal.
Second, the target range migration trace of a wide beam SAR is highly curved due to the wide synthetic aperture angle. In range compressed data, it is difficult to extract a target signal directly or after a simple data deskew process. Therefore, identifying prominent targets or estimating phase error cannot be efficiently performed in the range compressed data set.
Third, the strength of the prominent targets in GPR is usually not as high as other high frequency SAR. This leads to the need for highly sensitive phase error estimation algorithms, such as maximum likelihood estimation or contrast optimization.
All these factors indicate the need for an effective GPR autofocus method.